Electro-static discharge (ESD) or electrical overstress (EOS) is a significant problem in integrated circuit design, especially for devices with high pin counts and circuit speeds. ESD refers to the phenomena wherein a high energy electrical discharge of current is produced at the input and/or output nodes of an integrated circuit (IC) device as a consequence of static charge build-up on the IC package. The static charge build up can result from handling of the IC device by a human body or from handling by IC device manufacturing equipment. Electrostatic discharge has the potential to disable or destroy an entire device, at worst, and to decrease the device's reliability at best.
In general, smaller scale metal oxide semiconductor (MOS) devices with thinner gate oxides are more sensitive to ESD than larger scale devices with thicker oxides. For MOS devices having 1 micron or smaller geometries, discharges of approximately 1.5 amperes can damage or even destroy gates if adequate ESD protection is not provided. Modern flash memory devices with small cell sizes, including traditional floating gate and SONOS memory devices, are highly vulnerable to ESD and require sensitive and fast acting ESD protection circuits.
A variety of ESD protection circuits are available. These circuits can be based on small diodes, Zener diodes, bipolar junction transistors, and/or field effect transistors (FETs). The circuits can be connected between input or output pins and Vcc or Vss power supply pins. Power supply clamps, such as Vcc/Vss clamp structures, are also available. However, there remains an unsatisfied need for more sensitive and faster responding ESD circuits.